In preliminary experiments, we found that rats developed a transient hyperthermia immediately after acute exposure to low-level pulsed microwave irradiation (2450 MHz, 2 Mus pulse, 500 pps, 1 mW/cm2, SAR 0.6 W/kg). Continuous-wave microwave exposure at the same average incident power density failed to cause a similar effect. Thus, pulsed microwave-induced postexposure hyperthermia can be attenuated by treating the animals with narcotic antagonists, serotonin antagonists, and prostaglandin synthesis blockers. We propose to use this reliable and easily measurable effect of microwaves as an end point for comparison of the effects of the different parameters of microwave irradiation including power density and duration of irradiation. Effects of circularly versus linearly polarized microwaves will also be compared. Experiments are also designed to further explore the neural mechanisms of the postexposure hyperthermia. Possible involvement of the pulsed microwave-induced auditory effect in the hyperthermia will be studied by cochlear lesion. Involvements of central and pituitary Beta-endorphin will be studied by arcuate nucleus lesion, hypophysectomy, and dexamethasone treatment. Effects of microwave exposure on levels of Beta-endorphin in plasma, the pituitary gland and the brain will also be studied. In another preliminary experiment, we found that acute exposure to pulsed microwaves attenuated amphetamine-induced hyperthermia in the rat. This effect can be blocked by naloxone and is classically conditionable. The conditioned effect is also naloxone-blockable. Experiments are proposed to investigate whether the effects of microwaves on apomorphine-hypothermia and stereotypy, ethanol-hypothermia, and morphine-induced catalepsy and lethality are also blockable by narcotic antagonists and classically conditionable. It is expected that the results from the proposed research will further understanding of the neurological mechanisms mediating the effects of acute and repeated exposure to low-level microwaves.